Risk Assessment of NPP Level2 PSA

1. Risk Assessment of NPPLevel2 PSA JNES-VARANS Review Meeting – 2011. 6. 10

2. Risks from DBEs and BDBEs • Overview of Level2 PSA • Level2 PSA Procedure

3. Risks from DBEs and BDBEs • Risk = Occurrence probability × Consequenc ●Unplanned Shutdown ●Anticipated Operational Transient Probability ●Accident ●Core Damage・・・・・・・・・・・・・・・・Level-1 PSA ●FP Large Release・・・・ ・ ・ ・・Level-2 PSA ●Fatality, Evacuation ・・Level-3 PSA Magnitude of Damage

4. Overview of Level2 PSA Level2 PSA Procedure Investigation of Plant design and characteristic Level1 PSA PDS Classification and quantification of frequency for each PDS Define modes for containment failure Accident sequence analysis Accident progression analysis Quantification of frequency of containment failure RC classification and quantification of frequency for each RC Source term analysis for each RC Uncertainty and sensitivity analysis Level3 PSA

5. Level2 PSA Procedure • Investigation of Plant design and characteristic Gathering the information which are needed to implement Level2 PSA • Design information • System Flow Diagram • Skeleton Diagram etc. • Operation • Tec-Spec • Operating procedure • Accident Managementetc. • Others • Dissection with design engineer and plant operator • Plant walk down

6. Level2 PSA Procedure • Plant damage state (PDS) Classificationand quantification of frequency for each PDS • Classify the core damage sequences into PDSs taken account of : • Similar characteristics regarding their impact on the containment • and the mitigation potential based upon the system failures postulated • Quantification of frequency for each PDS

7. Means to Debris Cooling Timing of Core damage Means to CV Heat removal Timing of CV Failure Sequence Primary Pressure PDS CV Spray ＴＥＩ TEI Alternative CV Heat removable Yes ＴＥＨ TEW,TEH,TEI TEH High （Early） （After Core damage） No ＴＥＷ TED,TEF,TEW,TEH,TEI TEW Alternative CV Heat removable ＴＥＦ No TEF No TED,TEF ＴＥＤ TED Yes （No） ＳＬＣ Before Core damage SLC No Ａ SLC,G Ｇ Latter G CV Spray SLW,SLH,SLI,SLC ＳＬＩ Ｓ G SLI Alternative CV Heat removable After Core damage （Yes） ＳＬＨ SLW,SLH,SLI SLH Middle No Ｔ ＳＬＷ SED,SEF,SEW,SEH,SEI SLW CV spray SLW,SLH,SLI,SLC ＳＥＩ Ｇ G SEI Alternative CV Heat removable Yes ＳＥＨ SEW,SEH,SEI SEH Early （After Core damage） No Ｖ ＳＥＷ SED,SEF,SEW,SEH,SEI SEW Alternative CV Heat removable ＳＥＦ No SEF No SED,SEF ＳＥＤ SED Latter （Before Core damage） （Yes） （No） ＡＬＣ ALC Low Before Core damage （No） Ｖ AED,AEF,AEW,AEH,AEI,ALC V CV Spray V ＡＥＩ Eraly AEI Alternative CV Heat removable Yes AED,AEF,AEW,AEH,AEI ＡＥＨ V AEW,AEH,AEI AEH After Core damage No ＡＥＷ AED,AEF,AEW,AEH,AEI AEW Alternative CV Heat removable ＡＥＦ No AEF No AED,AEF ＡＥＤ AED Level2 PSA Procedure • Example of Plant damage state

8. Level2 PSA Procedure • Define modes for containment failure • all sequences that are postulated to involve containment failure are classified into modes of containment failure ・ α：Steam Explosion (in-vessel) ・ θ：Overpressure failure before core damage ・ γ：Hydrogen burning and detonation ・ ε：MCCI (core-concrete reaction) ・ η：Steam Explosion (ex-vessel) ・ σ：Direct Containment Heating ・ μ：Shell attack ・ τ：Containment overtemperature failure ・ δ：Overpressure failure after core damage ・ β：Failure of Containment isolation ・ ｇ ：Containment bypass; SGTR ・ ν：Containment bypass; IS-LOCA ・ φ：Intact Containment

9. Level2 PSA Procedure • Example for accident procedure(from PDS to CV failure mode) 8

10. Level2 PSA Procedure • Accident sequence analysis • Containment Event Tree (CET) provides a model for each PDS. • The CET addresses: • Recovery of mitigating systems and accident management actions with the objective of preventing of containment failure or mitigation of consequences. • Containment failure timing and release pathway that impacts the characteristics of fission product release to the environment • The potential for the occurrence of postulated phenomena in the containment that can impact containment failure or the release source term

11. Level2 PSA Procedure • Example of Containment event tree

12. Level2 PSA Procedure • Accident progression analysis • The accident progression analysis results are used as one of references for evaluation of the containment event tree analysis of the Level 2 PRA. • Perform phenomenological analysis to evaluate: • Occurrence timing of severe accident phenomena • Margin time for activation of severe accident mitigation features • Loading of the containment • Employ severe accident analysis code; • MELCOR • MAAP etc.

13. Level2 PSA Procedure • Quantification of frequency of containment failure • Quantify branch probability of CET for each PDS • For Severe accident phenomena • Refer to support analysis (accident progression analysis) • Engineering judgment Employ Decomposition Event Tree (DET) or Risk Oriented Accident Analysis Methodology (ROAAM) • For Severe accident mitigation features, including human actions • Employ Fault Tree • Engineering judgment

14. Level2 PSA Procedure • Release categories (RC) classification and quantification of frequency for each RC • Classify the Containment failure sequences into RCs taken account of : • Plant damage state • Containment failure modes • Release path to environment • Quantification of frequency for each RCs

15. Level2 PSA Procedure • Source term analysis for each RCs • Evaluate Source term for each RCs • Select the represented sequence for source term analysis • Frequency • Release value • Set up analysis condition • FP in the core • Gas or aerosol • Consistency between accident progression analysis • Period for Analysis

16. Level2 PSA Procedure • Uncertainty and sensitivity analysis • Uncertainty analysis • Consider the numerical uncertainty • Employ Monte Carlo simulation • Analyze Mean value and uncertainty range • Sensitivity analysis • Consider the modeling uncertainty; the key assumptions and key decisions that are made in developing the model.